Process for depositing a cadmium alloy protective coating on the back of a silver backed mirror



Nov. 28, 1961 H. I HEARD ET AL 3,010,841 PROCESS FOR DEPOSITING A CADMIUM ALLOY PROTECTIVE COATING ON THE BACK OF A SILVER BACKED MIRROR Filed Nov. 25, 1957 IMRRISM Mil/GS I/EHRJJ,

TIfEdJoRE DUMHS and cumyrmrl SIVERTZ BY: MM, M

RTfcRNEYS process and a backing agent which provide better United States atent fiice 3,010,841 Patented Nov. 28, 1961 This invention relates to a process for depositing coating on metallic surfaces and specifically for depositing a film of cadmium or of a cadmium-lead alloy on a me- .tallic surface. More particularly, it relates to the depositing of a stabilizing film of cadmium or cadmium-lead alloy on the silver surface of a silver mirror; it also relates to the backed silver mirror so formed.

In the formation of silver mirrors, the silver reflecting surface is normally. deposited from a solution of silver salts and reducing agents as a thin layer of free metal. The silver layer so formed must then be washed and dried, the mirror thus being handled in the usual manner. Such handling of the freshly silvered mirror without injury to the thin reflective surface becomes a serious practical problem. The layer is easily scratched and rubbed off. It was suggested placing a backing of copper electrolytically on the silver film but the supporting of the mirror plate in such processes was difiicult, time consuming and thereby costly.

It has also previously been suggested to protect the silver film against various spoiling agents by the galvanic plating of a copper film over the silver film. it was discovered, however, in many instances, that although the copper backed mirrors did offer protection to the silver surface against salt spray spoilage, there tended to be reduced adhesion between the silver film and glass. This often resulted in the combined copper-silver film lifting at the edge of the mirror when such edge was subjected to a necessary grinding operation.

Accordingly, it is an object of the present invention to disclose an improved method for the protection of the silver surface of a silvered mirror against the action of various spoilage agents.

It is a further object of this invention to disclose a backing agent for silver mirrors which improves the adhesion of the silver to the glass, especially when such mirror is to be subjected to an edge grinding operation.

It is a further object of this invention to disclose a pro tection against chemical spoilage agents.

These and other objects of the present invention are achieved in the process of applying to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying "an aqueous solution of cadmium nitrate in admixture with a water soluble lead salt selected from the group consisting of lead acetate and lead nitrate, generally in the direction of the silver backed mirror being coated, spraying an aqueous dispersion of afinely divided metal selected from the group consisting of zinc, chromium and manganese, generally in the direction of the silver backed mirror being coated, whereby said dispersion and said solution are caused to intermingle, and causing the intermingled dispersion to contact the silver backed mirror being coated. Thus, the process of the present invention I I anion is not especially reducible under the conditions employed. Satisfactory salts for this purpose include cadmium sulphate, cadmium acetate, cadmium borofluoride and cadmium nitrate when used under the conditions described hereinunder. Thus, it was found that, although cadmium nitrate when used alone gives useful results, it is preferred to use cadmium nitrate in conjunction with small quantities of lead nitrate. The proportions of the cadmium salts which may be used are not especially critical, but it is preferred that a minimum of .about 5.0 grams per liter to a maximum of about 15.0 grams per liter be used. Most desirably it is advantageous to use 5 .0 grams of the cadmium salt per liter of solution. While limits of 5.0-15.0 grams per liter have been arbitrarily placed on the carrying out of this invention, it is possible to effect the reaction with quantities differing slightly from the minimum and maximum values. Thus, the plating rate becomestoo slow when the amount of cadmium salt is less than 5 grams per liter. Also, when using cadmium salts in excess of 15 grams per liter, apparently the thickness of the cadmium or cadmium-lead alloy does not increase in .the manner expected for the increased concentration, and hence material is wasted. If it is desired to form a cadmium-leadalloy backing, a lead salt should be used in conjunction with the cadmium salt. Suitable lead salts include any water-soluble lead salt, such as for example, lead acetate or lead nitrate. Such salts may be used in an amount of from a minimum of about 0.1 gram per liter to a maximum of about 1.0 gram per liter, with a preferred quantity being 0.1 gram per liter.

The pH of the cadmium salt solution is only critical insofar as the rate of deposition is dependent upon the pH. Under normal circumstances, the pH of that solution would be between about 2.7 and about 5. A minimum pH of 2.7 is necessary because it was found, quite unexpectedly, that, at pHs lower than 2.7 the residual Zinc has a tendency to form agglomerates. In addition, at very low values of the pH, i.e. approaching 0, there exists a tendency to reduce the adhesion between the silver film and the glass. A low pH is, however, desirable since it is found that the plating rate increases as the numerical value of the pH decreases. Since such relationship between the pH and the plating rate does exist, an arbitrary maximum pH of 5- has been selected for practical purposes. Thus pHs above 5 slow down the plating reaction to a rate at which it becomes impractical for producing a film of cadmium or cadmium lead alloy in the time required for a conveyor type of operation, as envisaged by the present invention. Also, at pHs above 5 it becomes impractical for the etficient andoptimum use of the other chemicals in the solution. In adjusting the pH of the salt solution, it is necessary to add certain acids. It is advisable and preferable, although not necessary, that the solution contain an excess of acid sufiicient to remove the metal oxide film and still result in the pH necessary. Suitable acids for use in this process include glacial acetic acid, phosphoric acid, fiuoboric acid and sulphuric acid.

As a reducing agent in the reducing couple any metal having a suitably more positive standard potential than cadmium may be employed. In other words, those metals which app-ear above cadmium in the electromotive series of the metals are operative. It is thus possible to use metallic iron, chromium, zinc, manganese, etc. in finely divided dust form as a reducing couple. Thus, zinc dust, or a combination of zinc dust and iron dust suspended in distilled water has been successfully used. Although a suspension of this metallic dust in distilled water gives satisfactory results, it has been found to be desirable to add traces of the cadmium salt being used'and the acid used to obtain the correct pH, to the suspension of the metallic dust which is used as the reducing couple. 7

Thus, when cadmium sulphate is the salt and glacial acetic acid is the pH controlling agent, it is desirable to add traces of cadmium sulphate and glacial acetic acid to the zinc suspension.

It has been found that the most practical manner of efiectingthe coating of the cadmium or cadmium-lead alloy on the metallic surface is by means of spraying the salt solution and reducer solution simultaneously onto such surface. A most desirable technique in effecting such spraying is to use a fairly low pressure. A pressure of about 25 pounds per square inch has been found to be suitable.

In the preferred practice of the invention, the amount of the reducer metal powder in the aqueous suspension which is to be sprayed onto the surface being coated should be strictly controlled within the limits of about 5.0 grams per liter to about 25 grams per liter, in order to obtain the optimum rate of deposition of the cadmium or cadmium-lead alloy. It has been found that a method which comprises rapidly circulating the-suspension of the metal powder by means of a mechanical pump gives entirely satisfactory results. A diagrammatic representa- "tion of such system is shown in the figure.

As can be seen from the figure, the system is provided with a reservoir 10 equipped with a stirrer '12, and containing 11, which is either the aqueous suspension of a metallic powder such as zinc, or an aqueous suspension of a metallic powder such as zinc together with the aqueous solution of the cadmium salt to be coated. The

v solution and suspension is lead by conduit 13 to a pump 14 which pumps the material through conduit 16 to a pair. of spray guns 16. The pump and the reservoirwith the stirrer are so designed that the pressure of the soluthe system to assure that the pressure of the emerging mixture and the concentration of such mixture are at their optimum values. From the spray guns 16 a conduit 20 leads back to the reservoir 10, thereby completing the cycle of the continual circulating system.

Itis also seen from the drawing that the coating is applied to a silver mirror, which consists of a glass base 20 having a silver coating 22 thereon. The cadmium or "cadmium-lead alloy backing for the mirror is formed by the galvanic interactionbetween the metal salts and the reducer suspension, the reducer suspension acting through the silver film, and'is represented in the drawing by the reference numeral 23.

In order to compare the degree and type of protection afforded to the silver film of a silver mirror by the backing agents of the present invention, as contrasted to the backing "agents previously employed, various tests were carried out. V

One of the tests used was carried out on a standard ASTM salt testing machine. This standard salt test was used because it is the ofiicial measure of relative resistance of silver mirrors to spoilage agents. The test may be described in the following terms:

Several samples each of the mirror to be tested are inserted into the standard salt tester, the salt solution of which is sodium chloride GP. in distilled water at a temperature of 95 F. These samples are removed and inspected on the mirror side for signs of spoilage at intervals of 2, 5, and 20 hours.

Roof tests offer another set of spoilage conditions obviously not so reproducible as the salt test but important by way of confirming the latter. This test consists'of exposing sets of samples identical with those above used in the salt tests to natural weathering on a roof.

Another set of samples was tested for resistance to sulphur spoilage by applying strips of rubber known to contain sulphur on the back of the mirror samples. This rubber is widely used as shim material in assembling mirrors in wooden frames.

The following examples are given to illustrate the invention.

Example I copper, the material which has been heretofore used in affording such protection. The copper backed silver mirror was prepared following the process described in Canadian Patent No. 518,403.

This copper backed silver mirror and an unbacked bare silver mirror blank were subjected to the tests described hereinabove, and the results are as follows:

The bare silver showed blemishes between 2 and 5 hours, sometimes involving large areas. The copperbacked silver showed such blemishes by 5 hours and was usually extensively blemished by 10 hours. In the test for sulphur resistance, both the unbacked and the copper-backed silver mirrors turned black under the rubber after only an overnight exposure. a

The results show that a copper backed mirror gives satisfactory protection against salt spray spoilage but is unsatisfactory for resistance to sulphur spoilage. Thus, it is seen that, in general, the use of copper as a backing agent for silver mirrors is not entirely satisfactory.

Examples II-Vll These examples show the enhanced protection afforded by the use of the film of the present invention.

A series of reducer solutions and salt solutions for carryingout the process of the present invention were prepared in the following manner.

The reducer solution used in Examples II, III, IV and VI was prepared by dispersing 1 pounds of zinc dust in 10 Imperial gallons of water. The amount of Zinc dust is therefore 15 grams per liter. (Water" as used throughout this specification is intended to mean either deionized or distilled water.)

The reducer solution used in Examples V and VII was prepared by dispersing 2% pounds of zinc dust in 10 Imperial gallons of water. The amount of zinc dust isv therefore 25 grams per liter.

The salt solution used in Example II was prepared by dissolving 0.05 pound of cadmium sulphate in a minimum amount of water. 0.2 ounce of glacial acetic acid was added thereto and the mixture was diluted to 1 Imperial gallon by the addition of water. The amount of cadmium sulphate is therefore 5.0 grams per liter.

The salt solution used in Example III was prepared by dissolving 0.05 pound of cadmium acetate in some water, adding 0.2 ounce of glacial acetic acid thereto, and diluting to 1 Imperial gallon by the addition of water. The amount of cadmium sulphate is therefore 5.0 grams per liter.

The salt solution used in Example IV was prepared by dissolving 0.05 pound of cadmium acetate in a small portion of water, adding 0.2 ounce of glacial acetic acid and 0.00l pound of lead acetate thereto, and diluting the solution to 1 Imperial gallon by adding water. The amount of cadmium acetate is therefore 5.0 grams per liter, while the amount of lead acetate is 0.1 gram per liter.

The salt solution used in Example V was prepared by dissolving 0.5 pound of cadmium nitrate in a small amount of water, adding 0.02 pound of glacial acetic acid and 0.001 pound of lead nitrate thereto, and diluting the solution to 1 Imperial gallon by the addition of water thereto.

The salt solution used in Example VI was prepared by dissolving 0.2 pound of cadmium acetate in a small portion of water, adding 0.2 ounce of glacial acetic acid and 0.001 pound of lead nitrate thereto, and diluting the solution to 1 Imperial gallon by adding water. The amount of cadmium acetate is therefore 5.0 grams per liter, while the amount of lead nitrate is 0.1 gram per liter.

The salt solution of Example VII was prepared by dissolving 0.5 pound of cadmium nitrate in a small portion of water, adding 0.2 ounce of glacial acetic acid and 0.001 pound of lead acetate thereto, and diluting to 1 Imperial gallon by adding water. The amount of cadmium nitrate is therefore 5.0 grams per liter, while that of lead acetate was 0.1 gram per liter.

Examples II and III consisted of silver mirrors having a backing, according to the present invention, of metallic cadmium, while Examples IV, V, VI and VII consisted of silver mirrors having a backing, according to the present invention, of a cadmium-lead alloy. Each of the various examples was prepared using its respective salt solution and reducer suspension using the process and the apparatus fully described hereinabove with reference to the drawing.

The backed mirrors of the present invention were subjected to the same series of tests carried out for the backed mirror of Example I in order to determine the amount and degree of protection afforded by the film. The results are as follows:

When samples of the mirrors of Examples II-VII Were tested in the salt testing machine, it was found that such mirrors never showed blemishes under hours and usually showed only small blemishes by the end of 20 hours. In the Roof tests carried out using samples of the mirrors of Examples II-VII compared with a bare silver mirror, it was found that the mirror with silver only showed blemishes after two months. No sign of spoilage was noted on the cadmium backed sample. The mirrors of Examples II-VII showed no eifects after several weeks exposure to sulphur-containing rubber and so exhibit strong resistance to sulphur spoilage.

It is therefore seen from the above described tests that the silver mirror protected by a backing of metallic cadmium or a cadmium-lead alloy according to the present invention has a special capacity to resist spoilage agents of various kinds.

What we claim is:

1. A method of applying to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution of cadmium nitrate, in admixture with a water soluble lead salt Selected from the group consisting of lead acetate and lead nitrate generally in the direction of the back of the silver backed mirror being coated, spraying an aqueous dispersion of a finely divided metal selected from the group consisting of zinc, chromium, and manganese generally in the direction of the back of the silver backed mirror being coated whereby said dispersion and said solution are caused to intermingle, and causing said intermingled dispersion to contact the back of the silver backed mirror being coated.

2. A method of applying to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution, adjusted to a pH of about 2.7 to about 5, of cadmium nitrate, generally in the direction of the back of the silver backed mirror being coated, spraying an aqueous dispersion of a finely divided metal selected from the group consisting of zinc, chromium, and manganese generally in the direction of the back of the silver backed mirror being coated whereby said dispersion and said solution are caused to intermingle, and causing said intermingled dispersion to contact the back of the silver backed mirror being coated.

3. A method of applying, to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution, adjusted to a pH of about 2.7 to about 5, of cadmium nitrate, in admixture with a water soluble lead salt selected from the group consisting of lead acetate and lead nitrate generally in the direction of the back of the silver backed mirror being coated, spraying an aqueous dispersion of a finely divided metal selected from the group consisting of zinc, chromium, and manganese generally in the direction of the back of the silver backed mirror being coated whereby said dispersion and said solution are caused to intermingle, and causing said intermingled dispersion to contact the back of the silver backed mirror being coated.

4. A method of applying, to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution, adjusted to a pH of about 2.7 to about 5 by the addition of an acid selected from the group consisting of sulphuric acid, glacial acetic acid, phosphoric acid and fluorboric acid, of cadmium nitrate, in admixture with a water soluble lead salt selected from the group consisting of lead acetate and lead nitrate, generally in the direction of the back of the silver backed mirror being coated, spraying an aqueous dispersion of a finely divided metal selected from the group consisting of zinc, chromium, and manganese generally in the direction of the back of the silver backed mirror being coated whereby said dispersion and said solution are caused to intermingle, and causing said intermingled mixture to contact the back of the silver backed mirror being coated.

5. A method of applying to the back of a silver backed mirror by galvanic action, a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution of cadmium nitrate generally in the direction of the back of the silver backed mirror being coated, said aqueous solution being adjusted to a pH of about 2.7 to about 5 by the addition of an acid selected from the group consisting of sulphuric acid, glacial acetic acid, phosphoric acid and fluoboric acid, spraying an aqueous dispersion of a finely divided metal selected from the group consisting of zinc, chromium and manganese, generally in the direct-ion of the back of the silver backed mirror being coated whereby said dispersion and said solution are caused to intermingle and causing said intermingled dispersion to contact the back of the silver backed mirror being coated.

6. A method of applying to the back of a silver backed mirror by galvanic action a thin coating film of a cadmium-lead alloy such method comprising, spraying an aqueous solution of cadmium nitrate in admixture with a water soluble lead salt selected from the group consisting of lead acetate and lead nitrate generally in the direction of the back of the silver backed mirror being coated, spraying an aqueous dispersion of finely divided zinc generally in the direction of the back of the silver backed mirror being coated, whereby said dispersion and said solution are caused to intermingle and causing said intermingled dispersion to contact the back of the silver backed mirror being coated.

7. A method of applying to the back of a silver backed mirror by galvanic action a thin coating film of a cadmium-lead alloy, such method comprising spraying an aqueous solution of cadmium nitrate in admixture with a water soluble lead salt selected from the group consisting of lead acetate and lead nitrate generally in the direction of the back of the silver backed mirror being coated, said aqueous solution being adjusted to a pH of about 2.7 to about 5 by the addition of an acid selected from the group consisting of sulphuric acid, glacial acetic acid, phosphoric acid and fluoboric acid, spraying an aqueous dispersion of finely divided zinc generally in the direction of the back of the silver backed mirror being coated whereby said dispersion and said solution 8. A silvered mirror hearing, as a backing, a thin coating of a cadmium-lead alloy. 5

References Cited in the file of this patent,

UNITED STATES PATENTS 1,647,857 Colbert et a1, Nov. 1, 1927, 10

8 Snyder; Aug. 25', 1936 Qxieneau et'al. Apr, 8 1941 Harris Aug 18, 1942 Dragert Sept. 22-, 1942 Mccurtcheon June 21, I949 Meth Dec. 29, 1953 281111 Nov. 9, 1954 Hilemin Aug. 21, 1956 Nieter Feb. 26, 1957 

1. A METHOD OF APPLYING TO THE BACK OF A SILVER BACKED MIRROR BY GALVANIC ACTION, A THIN COATING FILM OF A CADMIUM-LEAD ALLOY, SUCH METHOD COMPRISING SPRAYING AN AQUEOUS SOLUTION OF CADMIUM NITRATE, IN ADMIXTURE WITH A WATER SOLUBLE LEAD SALT SELECTED FROM THE GROUP CONSISTING OF LEAD ACETATE AND LEAD NITRATE GENERALLY IN THE DIRECTION OF THE BACK OF THE SILVER BACKED MIRROR BEING COATED, SPRAYING AN AQUEOUS DISPERSION OF A FINELY DIVIDED METAL SELECTED FROM THE GROUP CONSISTING OF ZINC, CHROMIUM, AND MANGANESE GENERALLY IN THE DIRECTION OF THE BACK OF THE SILVER BACKED MIRROR BEING COATED WHEREBY SAID DISPERSION AND SAID SOLUTION ARE CAUSED TO INTERMINGLE, AND CAUSING SAID INTERMINGLED DISPERSION TO CONTACT THE BACK OF THE SILVER BACKED MIRROR BEING COATED. 